Process for cooking wood chips by the alkaline process of pulp manufacture



Patented Oct. 9, 1928.

' UNITED STATES 1,687,076 PATENT OFFICE.

ROBERT WOODHEA'D, OF HOPEWELL, VIRGINIA, ASSIGNOR TO VENNING D. SIMONS,

OF CHICAGO, ILLINOIS.

PROCESS FOR COOKING WOOD CHIPS H Drawing.

This invention relates to a new and improved p'rocess for cooking wood chips by the alkaline process of manufacturing paper pulp in soda and sulphate pulp-mills and the improvement is a licable whether the liquor used in the coo ng of the wood chips be a solution of sodium hydrate and other sodium salts, in which case the method employed is known in the trade as the soda process, or

whether the liquor used in the cooking of the chips be a solution of sodium hydrate, sodium sulphide and other sodium salts, in which case the method is known and understood as the sulphate process.

Attempts have heretofore been made to impregnate the wood chips prior to the cooking thereof with strong so-called white liquor composed of sodium h drate and other sodium salts, as in the so a process, or with so- 0 dium hydrate, sodium sulphide and other sodium salts, as in the sulphate process, hydrostatic pressure being applied in some cases for a period of from one-half hour to an hour. However, on account-of the drastic action of 2 these strong solutions on the cellulose, the strength of the manufactured pulp sufi'ered materially and a further disadvantage arose from the fact that the volume of liquor in the digester during the cooking process was too small, making it necessary to pump into the digester, while it was under pressure, eno h extra white liquor to enable the digester to e blown clean of cellulose by the action of pressure at the conclusion of the cooking process. It has also been attempted to steam the wood chips prior to cookin with the intention of removing the acid bodies ori inally present in the chips or formed during t e early stages of the cooking process, through the condensate 40 obtained from this preliminary steaming and, while this was accomplished to some exten the chips remaining in the digester after the steaming operation contain very much more moisture than they did initially and therefore reduce the densit of the cookin liquor while also providing a ditional work or the evaporator and the recovery department of the process. It was therefore found that instead of reducing the cost of (pulp manufacture, this practice materially a do to the cost and the results obtained were little better than those obtained by the regular methods previously employed.

BY THE ALKALINE PROCESS OF PULP MANU- FACTUBE.

Application filed October 12, 1927. Serial No. 225,854.

The principal object of the present invention is to provide an improved process which will leave the cellulose in a much stronger condition than that obtained by {the regular method of cooking, will insure a reduction of steam used in the evaporation of the used liquors in the process of recovery, effect a reduction in the cost of chemicals per pound of cellulose produced, reduce the equipment heretofore necessary in this department of a soda or a sulphate pulp mill, and reduce the maintenance cost in this department of the plant, without efiectin any increase in the time of the digester coo 'ng cycle.

The nature of the present invention will now be more fully described while at the same time comparingiit with the re ular method of cooking wood 0 ips as heretofbre practiced'in soda and sulphate pulp mills.

According to the usual method of cooking, as heretofore practiced, the digester is filled with wood chips and white liquor,consisting of a solution of sodium hydrate and other salts in the soda process and a solution of sodium hydrate, sodium sulphide and other salts in the sulphate rocess, is then added to the wood chips in t e digester, this white liquor containing the required amount of active alkali in the form of sodium hydrate in the soda process or sodium hydrate and sodium sulphide in case the sul hate process is employed. If, for example, ve hundred cu-. bic feet of white liquor are required to cook the wood chips to cellulose, it will be found that by this method this volume is not sufiicient to blow the cooked contents of the digester after cooking, and a quantity of previously used white liquor, known in the trade as black liquor, is therefore added to-the digester to make up the necessary volume, for example, one thousand cubic feet. After this addition of the black liquor, steam is admitted to thedigester and the cooking of the wood chips to wood cellulose takes place. 7

By the improved process of the present invention, the digester is first filledwith wood chips and there is then added, in preparation for a preliminary pre-heating or cooking, a quantity of previously used white liquor,

known as black liquor as stated above, to

which there has been added a small percentage of active alkali in the form of fresh white liquor. After adding the black liquor with the addition of the small percentage of white liquor, steam is turned on in the digester to raise the digester pressure to about to twenty or thirty pounds per square inch at which pressure it is held for approximately thirty minutes, therebeffecting a preheating or pre-cooking of t e wood chips in the digester.

The liquor in the digester is then blown into a tank leaving the preheated or pre-cooked wood chips in the digester in readiness for the final cooking operation. The fresh white liquor for the final cooking step is then introduc ed into the digester but, according to the improved method of this invention, the final charge of fresh White liquor is reduced from 25% to 40%, as compared with the previous practice, depending upon the amount of active alkali in the form of fresh white liquor which has been added to the black liquor in the preheating or pre-cooking charge of the digester. It is found that this saving in the quantity of white liquor required for the final cooking operation can be efiected by the introduction of a small quantity of fresh white liquor during the preheating process.

The volume of fresh white liquor employed in the final cooking operation depends entirel on the quantity of active alkali added to the lack liquor in the preheating or precooking operation, var ing usually between 25% and 40% of the v0 ume used in the regular method of cooking and this volume required'in the final cooking step of my process will be found to vary approximately as follows If no active alkali has been added in the form of fresh White liquor in the pre-heating or pre-cooking .ofperation, the white liquor required for the 'nal cooking operation will be 25% less than that required by the regular method of cooking heretofore practiced, and if from %to 1% of fresh White liquor has been added to the black liquor in the pre-cooking or pre-heating step, it will be found that 40% less volume of white liquor will be realuired in the final cooking operation than in e rior'practice of cooking if white liquor of t e same density be used.

Thus, by the improved-method of-the present invention, the quantity of wood chips which would require five hundred cubic feet of white liquor according to the prior method as in the example suggested above, would require only three hundred and. seventy-five cubic feet of white liquor in the final cooking step according to my improved process if no fresh white liquor were added in the preheating and pro-cooking step and if from to 1% of fresh white liquor were added to the black liquor during the preheating step of my process, only three hundred cubic feet of white liquor would be required in the. final cooking operation'hs compared with five hundred cubic feet of white liquor required in the sup osed example of cooking by the re metliod heretofore practiced. Since a V0 ume lar of one thousand cubic feet is necessary to effect the blowing of the cooked contents of the digester at the end of the cooking operation, in the example supposed above, it will be apparent that. by this improved method, this necessary volume can be made up by adding six hundred twenty-five cubic feet of black liquor where no white liquor is added during the pre-heating or pre-cooking step and seven hundred cubic feet of white liquor where /Z% to 1% of white liquor has been added during the previous pre-heating or pre-cooking step. Accordingly, there is a great saving in the use of fresh white liquor by my process while at the same time obtaining improved results in,the quality of the product.

The addition of a small quantity, preferably from to 1%, of active alkali to the wood chips prior to the final cooking operation not only to 40% in t e quantity of fresh white liquor necessarily used in the final cooking operation but it also results in an increase in tensile strength of the finished paper or paper board made from the cellulose obtained by this process, this increase in tensile strength being from 15% to 25%. There is also an increase in the yield of cellulose from the Wood chips ranging from 7% to 10%. The reduction in necessary equipment in the department where the soda and caustic alkali are recovered is about 25% as compared with former equipment and there is also a reduction of from 25% to 40% in the amount of steam used in the evaporators as compared with the amount of steam required by the prior practice. In addition, there is a reduceffects a reduction of from 25% tion in the maintenance cost of the recovery be practiced in various ways within the scope of the appended claims.

I claim:

1. The process of manufacturing paper pulp which comprises the operations of preheating wood chips in the presence of previously used digesting liquor to which a small quantity of fresh digesting liquor has been added, drawing off said liquor, and then effecting a final cooking of said wood chips in a digester.

2. The process of manufacturing paper pulp which comprises the operations of preheating wood'chips in a digester in the presence of a solution including a small quantity of fresh digesting liquor, drawing off said liquor from the digester, and then efiecting a final cooking of said Wood chips in the digester in the presence of a fresh digesting solution.

3. The process of manufacturing paper pulp which comprises the operations of preheating wood chips in the presence of previously used digesting liquor to which a small quantity of fresh digesting liquor has been added, drawing off said liquor, and then effecting a final cooking of said Wood chips in the presence of a solution consisting largely of fresh digesting liquor.

4. The process of manufacturing paper pulp which comprises the operations of preheating Wood chips in a digester containing black liquor from a previous manufacturing operation to which has been added a small quantity of fresh white liquor, drawing off said liquor from said wood chips, and then effecting a final cooking of said Wood chips in the presence of a solution made up largely of fresh white liquor.

5. The process of manufacturing paper pulp which comprises the operations of preheating Wood chips in a digester containing black liquor from a previous manufacturing process to which has been added from onehalf per cent to one per cent of fresh white liquor, continuing said preheating for about thirtyminutes underpre-ssure of from twenty to thirty pounds per square inch, then drawing off the liquor from said digester, adding fresh white liquor to the digester in sufficient quantity to eflect the final cooking of said wood chips, adding an additional quantity of black liquor suflicient to effect the blowing of the contents of the digester after cooking, and then effecting a final cooking of the contents of the digester.

In testimony whereof, -I have subscribed my name.

ROBERT WOODHEAD. 

